Lower-frequency radars, e.g., those operating in the MF, HF, and VHF bands, are useful for a number of applications. Among them are ocean surface current mapping and wave monitoring. Such radars are also well suited for detection of discrete targets, e.g., aircraft, ships, missiles, etc. Some advantages of such radars are:
(i) the ability to see beyond the horizon, in both skywave and surface-wave propagation modes; PA1 (ii) the radar wavelength being of comparable size to scattering target dimensions, resulting in resonance with the target; and PA1 (iii) the lower data rates (resulting from the low frequency) of such radars permit easier digital signal generation and processing.
These lower-frequency radars typically operate at frequencies three orders of magnitude less than the much more common microwave radars. The disadvantages of such low frequency radars compared to microwave radars primarily arise from the larger antenna sizes required in order to achieve pattern gains comparable to that of microwave systems. This is because the antenna sizes can be as much as three orders of magnitude larger than those required for microwave radars. A disadvantage of this antenna size requirement is that the antennas become prohibitively costly or too impractical for most applications. In contrast, if the antenna size is reduced, standard beam forming techniques provide very poor target angle resolution. In addition, inadequate target detection sensitivity can result due to the lower antenna gain.
Thus, one is faced with a decision: either incur the cost and difficulties of building a large antenna to obtain the target resolution and sensitivity afforded by the use of beam forming techniques and the high antenna gain, or utilize a smaller antenna and accept the resulting degradation of resolution and antenna gain.
A standard method by which a microwave radar determines target direction is to form a narrow beam. Location of a target within the beam places a constraint on the direction of the target, i.e., it must fall within the angular region defined by the beamwidth. This is why a narrow beam is so desirable: it reduces the uncertainty concerning the direction of the target. However, a high degree of angular resolution (i.e., a small beamwidth) is possible only when the antenna aperture is many wavelengths in extent. This is because the beamwidth (in radians) is on the order of the radar wavelength divided by the aperture dimension. As a result, when beam forming techniques for direction finding are used with HF skywave radars, phased array antennas several kilometers in length are required. Narrow-beam surface-wave radars, such as the British OSCR used for ocean current mapping use phased array antennas that require more than 100 meters of lineal coastal access, a frequently impractical constraint. HF antennas with size on the order of a wavelength (e.g., 10-20 meters) have nearly omni-directional patterns, and are considered inadequate for accurate radar angle determination if beam forming and scanning are employed due to their limited angular resolution.
An alternate way to determine the direction angle of a target is to employ direction-finding (DF) principles. In DF, the signals from the individual antenna elements are processed using an algorithm that estimates the angle(s) of arrival. In beam forming/scanning, the signals from the separate elements are combined to form and scan a beam; the estimated target angle then corresponds to the beam position where the combined signals are a maximum. However, this technique has rarely been used with radars. When antenna sizes are manageable, as they are at microwave wavelengths, beam forming/scanning is preferred to DF because it offers greater sensitivity for a given angular accuracy.
U.S. Pat. No. 4,172,255 describes a three-element HF radar receive array with half-wavelength inter-element spacing and gives a closed-form DF algorithm for mapping ocean surface currents. U.S. Pat. Nos. 3,882,506, 4,433,336, and 5,361,072 describe HF radar hardware implementations of two crossed single-turn air-loops and a monopole all mounted along the same axis. This forms a very compact antenna system. The latter three-element antenna configuration has been used with both a closed-form algorithm (where the loop signals are proportional to the sine and cosine of the bearing), and with a least-squares algorithm that searches for the optimum bearing angles by finding the minimum between a model for the signals received by the antennas and the actual measured data.
However, a disadvantage of the least-squares method is that the bearing is a nonlinear function of the measured/model signal amplitudes, so that a numerically inefficient search algorithm is required. When used to determine more than two bearings, this multi-dimensional grid-search process is prohibitively inefficient for use in real-time digital radar signal processing applications. Another disadvantage is that the least-squares algorithm does not allow for a robust, objective hypothesis-testing method to determine the number of signals at the same frequency from different bearings. This is because the required data covariance matrix among the antenna signals is nearly singular.
MUSIC (for MUltiple SIgnal Classification) is a direction-finding algorithm described in the article entitled "Multiple Emitter Location and Signal Parameter Estimation", by Ralph O. Schmidt, IEEE Transactions on Antennas and Propagation, vol. AP-34, no. 3, March, 1986. The algorithm has found usage in the signal intelligence gathering community, where it is used to determine the directions of non-cooperative radio emissions. In most such applications, antennas connected to separate receive channels are placed at convenient locations on the body of aircraft or ships. From the received signals, the algorithm locates the angles of arrival with respect to the vessel. The MUSIC algorithm has been used for locating the time delay of radar targets in the process of forming two-dimensional images for radar cross section ranges. However, the algorithm has not been used in radar applications for target angle determination, where beamforming/scanning has by far been the preferred approach for obtaining target direction. One reason for this preference is that the MUSIC algorithm requires signals that decorrelate rapidly, so that multiple target angles can be resolved; hard target signals normally do not have this property.